Lecture 13

PHYS 183 – The Milky Way Inside and Out – Tracy Webb Winter 2013
Lecture 13: February 4 , 2013h
 the Doppler effect  measuring velocity
o this is how we know the earth is expanding
o this is also how police know that you are speeding  uses light waves
 we hear different sounds because they have different frequencies
 when trains are stationary, the whistle you hear is in equal wavelengths. when the train is moving
forward, it is catching up between wavelengths being emits, so the sound waves are squished (smaller
wavelengths) and trailing behind has larger wavelengths  this is why you hear different
pitches/frequencies depending on where you are in relation to the train
 the same thing happens with light since it is a wave
 blueshift happens when they are shifted to shorter wavelengths (in front of train)
 redshift happens when they are shifted to longer wavelengths (behind train)
 intensity/brightness of the light changes in a light bulb, but the volume doesn’t change
 this happens at all wavelengths
 we measure the shift through the location of spectral lines  can tell how faster it is moving by the
amount of shift
 only measures the velocities along the line of sight  tells us the motion of an object toward or away
from us, not perpendicular *important point
o if it’s not moving directly away from you, you get l